Usually when we multiply two binomials, we get four terms and then two of them are "like terms" that we combine to get a trinomial. Here we look at a special situation where our middle terms are additive inverses, so our result is actually only two terms. This special case is called "a difference of perfect squares" because both of the two terms in our answers will be square terms (meaning, they both have square roots.) If you look at the original problem, both binomials will have the same terms, separated by one addition and one subtraction sign.